1. Field of the Invention
The present invention relates to an apparatus for producing hydrogen-dissolved water used as wash water or immersion water in a manufacturing process of semiconductor devices.
2. Description of the Related Art
In recent years, higher integration in a VLSI (very large scale integration) and higher fineness in wiring have been achieved. Consequently, in order to increase the degree of integration per unit area, various techniques have been developed for improving flatness of the surface of the substrate and for providing multi-layer wirings, and low resistance materials are being used for the wirings in order to realize high fineness for the wirings.
Most of the components of an LSI are formed on a silicon substrate. A typical manufacturing method of the LSI includes the following steps. Namely, the manufacturing method includes an oxidizing step for forming an oxide film in a high temperature diffusion furnace over the surface of a silicon wafer which has been polished in a mirror-like manner; a photoresist applying step for applying a photoresist (a photosensitive agent) over the insulated film to introduce photosensitivity; an exposing step for covering the wafer with a mask onto which a predefined pattern is drawn and for irradiating light for exposing the photoresist onto the wafer through the mask to print a pattern identical to the pattern drawn on the mask; a developing and etching step for removing the exposed portion only of the resist using a developing agent and immersing the wafer into an etching solution or the like to etch the insulated film of the exposed portion; an oxidizing and diffusing step for injecting an impurity into the silicon surface exposed as a result of the developing and etching step; and a metallizing step for creating a metal film layer for forming a wiring over the wafer surface. When a multi-layer wiring scheme is employed, another insulated film is created and then steps similar to those described above are repeated.
A wash step after a polishing step which is performed to flatten the substrate surface is very important in order to prevent wiring defects caused by residual impurities. Those impurities which are to be removed primarily include the residual abrasive grains of the polishing step. In order to remove these impurities, conventionally, high-purity water has been used in the main. In recent years, in addition to controlling the number of particulates remaining on the substrate surface after the washing step, the size of residual particulates is also controlled. In other words, due to the recent demands for higher degree of integration of the device itself and improvements in the performance of inspection apparatuses for the devices, the size of residual particulates is now controlled more strictly at a finer size level. The particulates to be controlled nowadays include those having a diameter of 0.12 μm or less.
With such increase in the cleanliness of semiconductor devices, the use of wash liquids that have a higher washing capability than high-purity water are widely used. Wash liquids having a high washing capability includes electrolytically ionized water obtained through electrolysis of water, hydrogen-dissolved water obtained by dissolving hydrogen gas into high-purity water, an alkali solution to which a chelate agent is added, and so on. Of these, the first two wash liquids have attracted attention as wash liquid having less environmental impacts because the amounts of additives to these wash liquids are very small and it is quite safe to handle these wash liquids.
For the electrolytically ionized water, by applying an electrolysis process to deionized water or high-purity water, anode water which is acidic is obtained in an anode chamber and cathode water which is reductive is obtained in a cathode chamber. By adding an acid, in particular, a hydrochloric acid, to the anode water, it is possible to obtain water quality which is acidic and wherein the oxidizing capability is higher. Similarly, by adding a base to the cathode water, it is possible to obtain water quality which is alkaline and wherein the reducing capability is higher. The anode water is used for removal of metals and sterilization, whereas the cathode water is used for removal of particulates and prevention of re-adhesion. The hydrogen-dissolved water is obtained by dissolving hydrogen gas into deionized water or high-purity water. As a method for dissolving hydrogen gas, typically, a method for contacting deionized water or high-purity water with hydrogen gas using a gas dissolving membrane in which a tube charged with hollow fiber membranes is widely employed because this method is highly efficient in dissolving hydrogen. Methods for supplying hydrogen include a method for using hydrogen gas used for semiconductor manufacturing, a method for using a hydrogen gas bomb, a method for using hydrogen gas generated through electrolysis of water, and so on. Of these, because of the cleanliness of the hydrogen gas and simplicity of equipment, the method for using hydrogen gas generated through electrolysis of water is often employed. The hydrogen-dissolved water is used for removing particulates and for preventing re-adhesion just like the cathode water of electrolytically ionized water.
In the manufacturing process of semiconductor devices, presence of any impurity is not desirable. Pollution by particulates causes pattern defects and degrades the reliability of insulated film. Impurities other than particulates include metals which cause degradation of the reliability of insulated film and various leak currents, organic substances which cause an increase in the contact resistance and degradation of the reliability of insulated film, inorganic ions, and unintentional formation of a natural oxide film. Because these impurities cause various defects, it is desired that these impurities be removed from the wash liquid to the maximum degree.
With the current technology, the quality of high-purity water that can be obtained by a typical high-purity water production apparatus used for manufacturing of an LSI of sub-micron design rule is, for example, as indicated in Table 1 shown below. With high-purity water having such quality, it is currently considered that no impurity attributed to the high-purity water adheres to the surface of the semiconductor substrate during a rinse treatment by the high-purity water.
TABLE 1ELECTRICAL RESISTIVITY18.2 MΩ · cm or greaterTOTAL ORGANIC CARBON1 μg C/liter or lessNUMBER OF PARTICULATES1/milliliter or less(particle size 0.05 μm or greater)NUMBER OF LIVE MICROBES0.1/liter or lessSILICA0.1 μg SiO2/liter or lessSODIUM0.005 μg Na/liter or lessIRON0.005 μg Fe/liter or lessCOPPER0.005 μg Cu/liter or lessCHLORIDE ION0.005 μg Cl/liter or lessCONCENTRATION OF HYDROGEN7.0ION (pH)OXIDATION-REDUCTION+350 mV (vs. NHE)POTENTIAL (ORP)CONCENTRATION OF DISSOLVED2 μg O/liter or lessOXYGEN (DO)
A high-purity water production device comprises a primary deionized water production device for obtaining primary deionized water by treating raw water with a coagulation and sedimentation device, a sand filter, an activated carbon filter, a reverse osmosis membrane device, a two-bed 3-tower ion exchange system, a mixed-bed ion exchange system, and a micronic filter, etc., and a secondary deionized water production apparatus for obtaining secondary deionized water by storing the primary deionized water in a primary deionized water tank and then treating the primary deionized water with an ultraviolet oxidation device, a cartridge polisher, and a membrane treatment device such as an ultrafiltration device and a reverse osmosis membrane device and so forth. By applying a secondary treatment to the primary deionized water, it is possible to remove residual particulates, colloidal materials, organic substances, metals, anions, etc. as much as possible to obtain high-purity water.
The high-purity water having the water quality as indicated in the above Table 1 is also used, for example, as the raw water for producing the electrolytically ionized water and hydrogen-dissolved water.
There is, however, a problem with the current high-purity water production scheme in that in the ultraviolet oxidation device used for achieving a desired value for the TOC concentration which is one of the parameters that must be controlled, hydrogen peroxide is generated albeit a very slight amount. In particular, in the ultraviolet oxidation device used for decomposing organic substances among the processing steps for the high-purity water, ultraviolet rays having a central wavelength of 185 nm are irradiated. In this process, the irradiated ultraviolet rays also decompose water molecules to produce hydrogen peroxide which is an oxidizing material. The amount of produced hydrogen peroxide is approximately 10 μg/L to several tens of μg/L, but the presence of the hydrogen peroxide sometimes causes unexpected oxidation during the manufacturing process of semiconductor devices.
The hydrogen peroxide continues to remain in the electrolytically ionized water or the hydrogen-dissolved water prepared from high-purity water containing a slight amount of hydrogen peroxide. In general, no reaction takes place between the slight amount of hydrogen peroxide and hydrogen gas within the high-purity water by merely mixing within a gas dissolving membrane unit. Although the electrolytic cathode water and hydrogen-dissolved water show reductive properties, the presence of a minute amount of hydrogen peroxide therein may cause unexpected oxidation to occur in the manufacturing process of semiconductor device.
As described earlier, in recent years, higher and higher fineness of semiconductor devices is being achieved. In addition to reducing the width of the wirings, reduction in the thickness of the wiring, insulated film, etc. are also desired as an essential requirement. As a substrate used for manufacturing of semiconductor devices, a silicon wafer is widely employed. In this case, a silicon oxide film is formed as the insulated film, and in some cases, the thickness of the silicon oxide film is controlled to the order of few nanometers. Because of this, the unexpected oxidation due to the presence of oxidizing materials in high-purity water can no longer be neglected.